some POSIX Bracket Expressions not working

For example when I'm trying to find and replace something using

g/[:digit:]/s/1/2/g

Vim showing error: Pattern not found: [:digit:], while the string contains digits.

Following POSIX Bracket Expressions are not working: [:digit:], [:xdigit:], [:upper:], [:lower:], [:print:], [:cntrl:]

At the same time all the others POSIX Bracket Expressions (like [:alnum:], [:alpha:], [:blank:] etc. work pretty well.

Vim version 7.4